1. Field of the Invention
The present invention relates to a contact unit for a switch, incorporating an elastic movable contact.
2. Description of the Prior Art
A switch employing a contact unit of this kind is closed when a force is applied through an actuator to an elastic movable contact to bring the elastic movable contact into contact with a fixed contact. The elastic movable contact is separated automatically by its elasticity from the fixed contact to open the switch when the force applied to the actuator is removed. Therefore the actuator can be returned to its initial position by the spring force of the elastic movable contact without using any special members for returning the actuator to its initial position. Thus, the switch has a simple construction and comprises a relatively small number of parts. A contact unit for a switch disclosed in Japanese Utility Model Laid-open (Kokai) No. 61-112536 employs a torsion coil spring as such an elastic movable contact, which is brought into contact with a fixed contact having the shape of a spring metal clip.
FIG. 6 is an exploded perspective view of a switch employing a prior art contact unit. This contact unit comprises a wafer 1 formed by molding and comprising a contact housing and a metal plate 2 combined with the contact housing by insert molding, a torsion coil spring 5 serving as both a movable contact and a return spring, an actuator 6 through which a pressure is applied to the torsion coil spring 5, and a cover 7 supporting the actuator 6 for axial movement and joined to the wafer 1. Terminals 3a and 3b projecting outside from the wafer 1, and a fixed contact 4 having the shape of a spring metal clip and standing upright within the wafer 1 are formed beforehand by pressing the metal plate 2. Although the terminal 3a connected to the fixed contact 4, and the terminal 3b with which the torsion coil spring 5 is brought into contact are connected outside the wafer as shown in FIG. 6, a portion of the metal plate 2 interconnecting the terminals 3a and 3b is cut off before completing the contact unit to disconnect the terminals 3a and 3b from each other.
One end of the spring wire forming the torsion coil spring 5 is extended substantially perpendicularly to the axis of the coiled portion 5a of the torsion coil spring 5 to form an elastic arm 5b, the free end of the elastic arm 5b is bent in a direction parallel to the axis of the coiled portion 5a to form an engaging portion 5c that engages the actuator 6, and the free end of the engaging portion 5c is bent toward the coiled portion 5a to form a contact portion 5d. When assembling the torsion coil spring 5, the wafer 1 and the actuator 6, the coiled portion 5a is put on the contact housing of the wafer 1 with the contact portion 5d located obliquely above the fixed contact 4 and the coiled portion 5a in contact with the metal plate 2 so that the torsion coil spring 5 is connected electrically to the terminal 3b, and the actuator 6 is put on the wafer 1 with its actuating part 6a seated on the engaging portion 5c of the torsion coil spring 5. The cover 7 has a guide hole 7a for guiding the actuator 6, and a holding portion 7b for holding the coiled portion 5a of the torsion coil spring in place on the wafer 1.
In this prior art contact unit, when no pressure is applied to the actuator 6, the actuator 6 is held at an upper position by the spring force of the torsion coil spring 5, and the contact portion 5d is separated from the fixed contact 4 as shown in FIG. 7(a), so that the terminals 3a and 3b are disconnected to open the switch.
When pressure is applied to the actuator 6, the actuating part 6a depresses the engaging portion 5c to bend the elastic arm 5b elastically so that the contact portion 5d is moved obliquely downward. When the actuator 6 is depressed by a predetermined distance, the contact portion 5d is forced to enter between the contact tongues 4a of the fixed contact 4 to connect the terminals 3a and 3b through the fixed contact 4 and the torsion coil spring 5, so that the switch is closed.
When the pressure is removed from the actuator 6, the elastic arm 5b of the torsion coil spring 5 restores its original shape automatically, the contact portion 5d is disengaged from the contact tongues 4a of the fixed contact 4 to open the switch and the actuator 6 is raised to its initial position by the engaging portion 5c as shown in FIG. 7(a).
In this prior art contact unit, even a slight dislocation of the torsion coil spring 5 is liable to cause imperfect contact between the coiled portion 5a of the torsion coil spring 5 and the metal plate 2, because the torsion coil spring 5 is connected electrically to the terminal 3b by holding the coiled portion 5a put on the wafer 1 in contact with the metal plate 2. Thus, the electrical connection of the torsion coil spring 5 serving as a movable contact and the terminal 3b is not highly reliable. Furthermore, the assembly of the torsion coil spring 5 and the wafer 1 requires troublesome assembling work particularly when the contact unit is designed for a small switch.